1. Field of the Invention
The present invention relates to an SOA (Semiconductor Optical Amplifier) array optical module, and more particularly, to an SOA array optical module using a lens array for controlling a plurality of optical signals by arranging a plurality of optical gates in each of which a lens array is provided between an SOA array and an optical fiber array.
2. Description of the Related Art
In recent years, the distance and the capacity of an optical communications network have been increasing with a growing demand for a communication made by a broadband service.
For example, high-speed and large-capacity WDM (Wavelength Division Multiplexing for simultaneously transmitting a plurality of signals with a single optical fiber by multiplexing light beams of different wavelengths) is currently under development.
In the meantime, a higher-speed, larger-capacity and flexible optical communications network has been demanded with the rapid popularization of the Internet, and an increase in the volume of data traffic.
An optical packet switching technique draws attention as a technique for building such an optical communications network.
The optical packet switching is a technique for making packet switching by using communication information totally unchanged as light. This technique eliminates restrictions on an electronic processing speed in comparison with conventional switching for once converting an optical signal into an electric signal. Therefore, the processing speed depends on only the propagation delay time of light, and accordingly, a high-speed and large-capacity transmission can be made.
If an optical signal is switched in units of packets, gate switches are used to turn on/off the optical signal. The gate switches for turning on/off an optical signal with an electric control include an electro-absorption gate switch and a semiconductor optical amplifier gate switch.
The electro-absorption gate switch is intended to change optical absorption by using electro-absorption effect. However, this gate has a disadvantage of a large loss even in a transmission state.
In contrast, the semiconductor optical amplifier gate switch is intended to change a gain with a driving current applied to a semiconductor amplifier, and has not only a function as an optical gate for turning on/off light but also an amplification function (to amplify and output light when a gate is turned on). Accordingly, this gate switch currently attracts attention as an optical element that reduces a loss of an optical signal and makes high-speed switching.
For an SOA, its extinction ratio of ON (open) to OFF (closed) of a gate is high, and its amplification mechanism can reduce an optical loss. Since the SOA is an optical element formed with a semiconductor, it has an advantage of downsizing enabled at low cost with a semiconductor integration technique.
The extinction ratio is a ratio of the average light intensity of signals “1” and “0” when a gate is ON to that of signals “1” and “0” when the gate is OFF. As the extinction ratio becomes higher, ON/OFF of a gate can be more explicitly identified. As a result, signal crosstalk that affects another port can be reduced, and a bit error rate becomes low.
FIGS. 1A and 1B schematically show two examples of optically coupled configurations of a conventional single-channel SOA module. Dotted-dashed lines shown in FIGS. 1A and 1B depict the propagation paths of an optical signal.
In the single-channel SOA module using a single lens shown in FIG. 1A, an SOA 2 is arranged between a single-channel optical fiber 1a arranged on an input side indicated by an arrow a, and a single-channel optical fiber 1b arranged on an output side indicated by an arrow b.
Additionally, a second lens 4a and a first lens 3a are arranged between the single-channel optical fiber 1a and the SOA 2 on the input side, whereas a first lens 3b and a second lens 4b are arranged between the SOA 2 and the single-channel optical fiber 1b on the output side.
Furthermore, hermetic windows 5a, 5b for hermetically sealing a semiconductor element (SOA) are arranged respectively between the second lens 4a and the first lens 3a, and between the first lens 3b and the second lens 4b. 
As shown in FIG. 1A, output light 6 of the SOA 2 is inclined by 22.3 degrees with respect to an axis 7 of the SOA 2. Therefore, optical coupling is made by arranging the first lens 3b and the second lens 4b so that their surfaces become vertical to the inclined output light of the SOA 2. This applies also to the input side.
The optical coupling system shown in FIG. 1A is a technique normally used to optically couple an optical module such as an LD (Laser Diode: semiconductor laser) that outputs a light beam vertically to an end face of an element.
For a gate switch using an SOA, an unnecessary oscillation made by return light must be prevented by reducing reflection on the light output end face of the SOA to 50 dB or less in order to prevent an oscillation caused by the internal reflection of the SOA.
To implement this, the end faces of the chip of the SOA 2 are initially inclined as described above so that a line vertical to the end face on the light output side of the SOA 2 and an optical waveguide within the SOA form, for example, an angle of 7 degrees.
Then, a return loss (reflected return light) is suppressed by outputting an optical signal to the first lens 3b as indicated by the dotted-dashed line, for example, at an angle of 22.3 degrees with respect to the vertical line (broken line) of the end face of the chip based on a relationship between the refractive index of the optical waveguide within the chip and that of space.
FIG. 2A shows the inclination of output light with respect to the axis when the end faces of the chip of the SOA are inclined, whereas FIG. 2B shows a relationship between the inclination (output angle) and the return loss ratio of the output light. In FIG. 2B, its horizontal axis represents the output angle θ (degrees), and its vertical axis represents the return loss ratio (dB).
According to the relationship between the inclination angle θ and the return loss ratio dB of the output light, which is shown in FIG. 2B, efficiency is normally regarded as being high on the whole by reducing the return loss ratio by −20 to −30 dB.
However, it is insufficient that reductions in the reflection on the light output end face by using the inclination angle θ is on the order of −20 to −30 dB. Therefore, the end faces of the chip are AR (Anti Reflection)-coated.
FIG. 3 shows a state where AR coated multilayer films are formed on the end faces of the chip of the SOA. As shown in FIG. 3, AR-coated multilayer films 8a and 8b are formed on both end faces of the chip of the SOA 2. In this way, the magnitude of a return loss is reduced at present by setting the light output angle of the SOA 2 and by forming AR coated layers.
Additionally, in a single-channel SOA module using spherical lensed fibers shown in FIG. 1B, the SOA 2 is arranged between the single-channel spherical lensed fibers 9a and 9b, which are arranged on input and output sides respectively indicated by arrows a and b.
The tips of the spherical lensed fibers 9a and 9b are formed as a sphere and have the action of a lens. Therefore, the first lenses 3a, 3b and the second lenses 4a, 4b, which are shown in FIG. 1A, can be omitted.
Since this configuration easily enables optical coupling for obliquely output light, it is used for an experiment of a research, etc. in many cases. However, this configuration has difficulty in hermetically sealing the SOA. Therefore, it is rarely used as a practical module.
If the SOA is hermetically sealed with the configuration shown in FIG. 1B, a hermetically sealing part cannot be provided between the SOA 2 and the spherical lensed fibers 9a, 9b. Therefore, the SOA 2 and the tips of the spherical lensed fibers 8a and 8b are to be hermetically sealed altogether.
As a result, reliability cannot be ensured due to a displacement, etc., which occurs between the SOA 2 and the spherical lensed fibers 9a, 9b with the thermal expansion and contraction of a hermetically sealed body based on a change in an ambient temperature. This is the reason why this single-channel SOA module using spherical lensed fibers is used only in laboratories and not put into practice use.
In the meantime, reductions in the size, the consumed power and the cost of a module or a device by collectively forming a plurality of channels, namely, by arraying a plurality of channels have been demanded for an optical gate switch.
In this case, the pitch of a semiconductor element array such as an LD array, an SOA array, etc., and that of an optical fiber array are an issue when optically coupling these arrays.
Known as an optically coupled and arrayed structure using spherical lensed fibers is a configuration where a semiconductor optical amplifier (SOA) and an external resonator using a fiber grating are combined, the tip of the fiber grating is made spherical, and the tip and the light output side of the semiconductor optical amplifier, on which a low reflection film is formed, are optically coupled and arrayed (for example, see Patent Document 1). However, Patent Document 1 does not disclose hermetical sealing.
Also known is a configuration where silica-based lightwave circuit substrate on which an SOA array as gate switches is implemented as a hybrid integrated circuit, and a silica-based lightwave circuit substrate on which an arrayed waveguide grating is formed are coupled to make optical signal switching such as optical cross-connect, etc. (for example, see Patent Document 2).
The above described Patent Documents 1 and 2, however, do not adopt a lens array. Normally, the diameter of an optical fiber in an optical fiber array is 125 μm, and many optical fiber arrays of 250-μm pitch are commercially available.
Also a semiconductor element array of 250-μm pitch is normally chosen, because the number of elements obtained per unit area of a substrate is desired to increase.
The pitch of an optical fiber array and that of a semiconductor element array are the same as described above. Therefore, if only a lens array of the same pitch as those of the arrays exists, it seemed to be possible to easily implement a configuration where optical gate switches of a plurality of channels are collectively formed by optically coupling a semiconductor element array and an optical fiber array via a lens array.
However, the diameters of the lenses (the first lenses 3a, 3b and the second lenses 4a, 4b) used for the optical coupling shown in FIG. 1A are approximately 1 to 2 mm in normal cases. Therefore, a lens array of 250-μm pitch cannot be formed by arranging lenses of this size.
Accordingly, an array implemented by forming many convex parts on a transparent substrate is normally adopted as a lens array used to optically couple a semiconductor element array and an optical fiber array.
FIG. 4 schematically shows an example of a well-known optically coupled configuration implemented by arranging a lens array between a semiconductor element array composed of an LD and an optical fiber array. Here, the LD is a light source.
The optically coupled configuration shown in FIG. 4 represents an example using the lens array 13 of 250-μm pitch for the optical coupling of the LD array 11 of 250-μm pitch and the optical fiber array 12 of 250-μm pitch.
In this example, the lens array 13 represents a micro-lens array implemented by forming convex lenses 15 of 250-μm pitch on the surface of a substrate 14 having a thickness of 500 μm.
The lens array 13 is arranged in parallel to the end face of the output side of the LD array 11 by orientating the surface, on which the convex lenses 15 are formed, to the LD array 11.
The optical fiber array 12 is arranged so that its input face, which is formed vertically to the axis of an optical fiber 18, becomes parallel to the rear face of the lens array 13.
An optical signal 17, which is output from a light waveguide 16 of each element of the LD array 11 along an optical axis 19 vertical to the end face on the output side of the LD array 11, is incident to the convex lens 15 of the lens array 13 along the optical axis of the convex lens 15, converged while passing through the lens array 13, and incident to each optical fiber 18 of the optical fiber array 12 along the optical axis of the optical fiber 18 after passing through the rear face of the lens array 13.
Namely, the optical axis of each of the elements through which an optical signal passes is formed by the linear optical axis 19 in the optically coupled configuration using the LD array 11.
The above described configuration where an optical signal from a light source is input to an optical fiber array via a lens array by using an LD array as the light source conventionally exists. However, a lens array has never been adopted in an optically coupled configuration of a gate switch for relaying and turning on/off an optical signal.
The arrayed gate switches are disclosed by the above described Patent Documents 1 and 2. However, neither of these techniques adopts a lens array.
Gate switches for turning on/off an optical signal mainly include an electro-absorption gate switch and a semiconductor optical amplifier gate switch. However, the electro-absorption gate switch has a disadvantage that a loss is large even in a transmission state as described above.
[Patent Document 1] Japanese Published Unexamined Patent Application No. 2000-236138
[Patent Document 2] Japanese Published Unexamined Patent Application No. 2003-149614